1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and, more particularly, to a magnetic resonance imaging apparatus for imaging information on spin density, chemical shifts, etc., of specific atomic nuclei existing within a local region of an object under examination.
2. Description of the Related Art
Magnetic resonance imaging apparatus is apparatus which acquires chemical and physical information of substances in the form of magnetic resonance signals and reconstructs an image from the signals by utilizing resonance absorption of radio-frequency magnetic field energy at specific frequencies by atomic nuclei having intrinsic magnetic moments and placed in a static magnetic field.
In living biological tissue there exist chemical substances having atomic nuclei, such as .sup.1 H, .sup.13 C, .sup.23 Na and .sup.31 P, which produce such magnetic resonance signals. These substances take part in the metabolism of the biological tissue. Especially, .sup.31 P compounds, such as PCr (creatine phosphate), ATP (adenosine triphosphate), ADP (adenosine diphosphate), etc., and .sup.31 P (phosphorus) existing in biological tissue as Pi (inorganic phosphorous) take part in various types of energy metabolism (acquisition, conservation and consumption of energy), The measurement of density ratios and density distributions of these substances is very useful in evaluating the physiological activity of biological tissues. The use of magnetic resonance imaging apparatus makes it possible to identify and quantitatively determine specific atomic nuclei, such as .sup.1 H, .sup.13 C, .sup.23 Na and .sup.31 P, for each of their compounds. By imaging those atomic nuclei as chemical shift images, a spatial distribution of each of the compounds can be obtained.
In diagnosis using a chemical shift image of a phosphorous compound, it is usual practice to display a proton image and a chemical shift image of a phosphorous compound and make a comparison between them for identification and quantitative determination of the phosphorous compounds. The detection sensitivity for magnetic resonance signals of phosphorous compounds is so low as to be about 1/10.sup.5 of that of protons. In order to acquire magnetic resonance signals of a phosphorous compound with required signal-to-noise ratios, therefore, it is necessary to increase the volume of a voxel. A chemical shift image of a phosphorous compound is small in the total number of voxels within an imaging region as compared with a proton image, resulting in reduction of spatial resolution.
This problem will be discussed taking, by way of example, chemical shift imaging which was actually conducted by the inventors of the present invention using a phantom. Such a phantom as shown in FIG. 6 was prepared. This phantom has an annular-type container 60 which is partitioned into four compartments 61, 62, 63 and 64, the rooms 61 and 63 being filled with a highly concentrated solution of phosphoric acid and the compartments 62 and 64 being filled with a lowly concentrated solution of phosphoric acid. In FIG. 12A there is shown a proton image obtained from a slice of the phantom, which is parallel to the drawing sheet, taking the phantom as a sample to be examined. In FIG. 12B there is shown a chemical shift image of the phosphorous compound which was obtained from the same slice as the proton image of FIG. 12A using the conventional imaging method. In order to indicate a correspondence in position between the images, grid lines (indicated by broken lines) representing an imaging matrix (data acquisition matrix) of the chemical shift image are displayed on the proton image as shown in FIG. 12A. A doctor performs identification and quantitative determination while making a comparison between the chemical shift image of FIG. 12B and the proton image of FIG. 12A.
As can be seen from FIG. 12B, however, the chemical shift image of the phosphorous compound is coarse and low in spatial resolution as compared with the proton image of FIG. 12A. The reason is that the volume of each voxel is made large as described above in order to make the signal-to-noise ratio high in obtaining magnetic resonance signals of a phosphorous compound. As a result, it becomes difficult to understand the positional relationship between a chemical shift image of a phosphorous compound and an object to be measured. Thus, there arises a problem that it is difficult to accurately understand the distribution of a phosphorous compound which is a candidate for chemical shift imaging.